Effect pigments are used to introduce metallic or pearlescent luster to a wide range of products such as paints, coatings, inks, and plastics. One property of the pigment flakes is that they tend to orient in parallel to the surface during application due to their high aspect ratios. Among the various effect pigments, aluminum pigments are widely used for their ability to provide high reflection of light at specular angle and render metallic effects on products.
Performances of effect pigments depend on their interaction with their matrixes. Efforts have been made to enhance pigment dispersion and compatibility in the pigment's application environments.
The surface of a pigment can be treated with surface active reagents that can be either physically absorbed onto the pigment surfaces or chemically bonded to the pigment surfaces. U.S. Pat. No. 6,761,762 discloses modifying the surfaces of the pigments by using a surface-modifying reagent which have one surface active group that binds to the pigment surfaces and at least one more functional group to adjust the interaction between the pigments and their applied media.
Pigment surface modification and encapsulation have also been extensively utilized to introduce barrier functions, protect the pigment from environmental attacks and extend the shelf-life of the pigments and weatherability of the pigmented objects. It is known that aluminum readily reacts with water and forms hydrogen and aluminum hydroxide, thereby leading to gassing and discoloration. As more and more stringent environmental regulations require lower volatile organic compound (VOC) contents, there is an increase in demand for the use of aluminum pigments in waterborne paints and inks. During the manufacture of such applications, aluminum pigments are dispersed in media composed of polymers, water, and other solvents, and under typical application conditions, pigmented materials are frequently exposed to basic and/or acidic environments.
Efforts have been made to protect the aluminum flakes with inorganic and organic materials. For example, corrosion inhibition reagents such as organic phosphate (U.S. Pat. Nos. 4,565,716, 4,808,231), organic phosphite (U.S. Pat. No. 4,808,231), vanadium compounds, and chromium compounds (U.S. Pat. No. 4,693,754) have been investigated to mitigate gassing of aluminum flakes. However, improvements in the protection of aluminum pigments for waterborne applications are desired. Moreover, metallic pigments for the most part do not disperse well enough in either aqueous or solvent borne systems. Aluminum flakes with good gassing stability can be obtained via silica coating (U.S. Pat. No. 2,885,366; 3,954,496). However, the brittle silica coating can be damaged by excessive mechanical stress. Furthermore, to reduce the agglomeration and to improve dispersion of pigment flakes, additional surface treatment may be necessary to achieve acceptable performances.
Efforts also have been made to encapsulate the aluminum flake with a polymer coating to enhance their compatibility with the polymeric binders and provide barrier function to protect pigments. For example, U.S. Pat. No. 7,479,323 discloses pre-treating aluminum flakes with polyvinyl alcohol to improve the dispersion of pigments in aqueous media. U.S. Pat. No. 4,213,886 discloses procedures to modify aluminum flakes with monoethylenically unsaturated silane. In such procedures, free radical polymerization between the silane and acrylic monomers results in the formation of polymer coated pigments.
To enhance the compatibility of the pigment to the binder, functional groups can be incorporated into the polymer coating. With organosilane or polymer coating, the interaction between the pigment and the binder can be adjusted by mixing silanes or tuning the composition of polymers. However, such coatings fail to provide satisfactory protection of the aluminum flakes against gassing. Moreover, the poor adhesion between the polymer and the pigment surfaces may result in loss of desired properties.
In general, polymer coated pigments that have been developed thus far suffer from two disadvantages. First, the coating may be easily damaged due to poor adhesion between polymer and the substrate underneath. Second, while the incorporation of functional monomers into the polymer chains may improve the dispersion/compatibility of pigments inside an applied media, these functional groups may adversely affect the chemical resistance of the flakes.
In previous approaches, polymer chains have been either physically absorbed or loosely bounded to the pigment surfaces. In the former case, the absorbed polymer chains can be easily replaced by other polymers and solvents. In the latter one, polymers are formed through radical polymerization of monomers in solution with the presence of vinyl or other alkenyl and/or alkynyl unsaturated functional groups on flakes. Once radicals of propagation polymerization chain ends react with surface bounded vinyl groups, the polymer chains are covalently bounded to the surfaces. In this case, because of steric hindrance, polymer chains anchored on the surface at the early stage of the polymerization may prevent the radicals of unbounded polymer chains in solution from reacting with surfaces, thus reducing the grafting density and therefore the thickness of the polymer coating. Furthermore, free radical polymerization used in these approaches offers limited control of the structure of the polymer coatings as only random copolymers can be synthesized.
Additionally, the pigments are usually applied to a substrate within a formulation where they are dispersed in polymeric resins. However, the interactions between the pigments and the polymeric resin are so complex that great efforts have been made to address the compatibility between the pigments and the resin systems within the formulation.